The parameter used for defining the combustion ratio is the factor λ which measures the excess air with respect to the stoichiometric ratio; λ=1 corresponds to the stoichiometric ratio, λ<1 indicates insufficient air, λ>1 indicates excess air.
The problem of controlling carburetion is particularly felt in small two-stroke engines, for example used for portable tools in the agricultural and woodlands industry, such as string trimmers, chainsaws and the like.
In the technical terminology, the expression “rich mixture” is used to indicate a mixture containing an excess amount of fuel, and the expression “weak mixture” is used to indicate a mixture containing an insufficient amount of fuel, where excess or insufficient amounts do not refer to the stoichiometric ratio, but to the desired correct combustion ratio compared to the conditions of use of the machine.
In practice the desired correct combustion ratio is never equivalent to the stoichiometric ratio, though very close thereto.
Both the maximization of the supplied power and the minimization of the polluting elements in the exhaust correspond to a correct combustion ratio in the desired conditions of use.
In small two-stroke engines, such as the ones used in portable tools, like chainsaws, parting tools, string trimmers or the like, it has been observed an optimal operating range corresponds to λ=0.85-0.95, i.e. a fuel mixture with slight air deficiency, i.e. a slightly “rich” mixture.
Values of λ lower than those indicated lead to a loss of power and excess exhaust fumes; greater values of λ instead lead to a hazardous overheating of the engine.
Thus, carburetion control is an essential activity for the sound operation of the internal combustion engine and several control systems are known.
Systems are known, for example described in document U.S. Pat. No. 6,029,627, which use the ionization current as the carburetion control parameter.
The ionization phenomenon starts within the combustion chamber where, due to the fuel oxidation reaction and due to the heat generated by the combustion, ions are generated.
In the presence of two differently charged poles arranged in the combustion chamber there occurs a migration of ions between the poles, referred to as the ionization current.
The use of the electrodes of the fuel mixture spark plug as poles is known.
The expression ionization current is used to indicate the current that passes between the two electrodes measured from outside.
The systems for measuring the current are known and thus they will not be described in detail.
The ionization current depends on several engine operation parameters and it may be considered as a function of two significant parameters, like the angular position of the crankshaft, i.e. the position of the piston in the cylinder, and the factor λ.
The angular position of the crankshaft is indicated by corresponding the top dead centre to 360°.
The ionization current diagram, as a function of the degrees of rotation of the drive shaft, has two peaks, the first due to the ions generated by the oxidation reaction (combustion) of the mixture and the second due to the ions generated by the amount of heat generated by the combustion.
The first peak always occurs, while the second peak occurs only when the engine generates a given power; thus it is not always available.
Said peaks are measured as a function of the degrees of rotation of the drive shaft, and the value thereof varies also as a function of λ.
For each value of λ we will thus have a given curve of the ionization current as a function of the degrees of rotation of the engine.
The ionization current diagram as a function of the factor λ has the peak for the value of λ close to 0.9 for small two-stroke engines.
The ideal carburetion occurs at the maximum peak of the ionization current as a function of the position of the drive shaft, this peak also varying as a function of λ; thus, the known methods provide for measuring—at each cycle—the value of the ionization current as a function of the position of the drive shaft and have a retroactive action on the value of λ to keep the value of the current close to the maximum value.
This method of operation is however difficult to apply in practice given that the variations of the value of the ionization current from the peak value are so low that they require very sensitive and sophisticated instruments, that are too expensive to use in the present field of application.
In document WO 98/37322 measurements are taken of the ionization signal (the ionization current) internally of the combustion chamber, and the factor λ is adjusted according to the signal.
The method for adjusting the λ uses the identification of a first and possibly a second peak in the ionization current and the maximisation of at least one of the peaks according to the ionizing current.
It can comprise a comparison between the peaks of current in the various cylinders of the engine.
The factor λ is modified by acting either on the butterfly valve or on the fuel injector.
The analysis of the ionization signal is done at the same time as the analysis of other significant parameters of the engine, among which the concentration of O2 in the exhaust.
When dealing with small two-stroke engines (page 13 from line 9), the document confirms that the method comprises maximizing the first and the second peak of ionization in all the operating conditions of the motor (page 9, lines 23, 24), or the determining of the value of λ at which there is a misfire detection.
Misfire detection is obtained by progressively weakening the mixture, up to identifying the value of λ that causes it; the functioning is stabilized thanks to a return to a reasonably enriched mixture.
Document WO2007/042091 describes a method in which it seems essential to construct, for each cylinder, a λ curve according to the ionization current between the unleashing of the spark and the end of the ionic phenomenon.
The document also includes determining (from page 4, line 15) the value of λ on the curve, and a corrective value of λ which varies according to the type of engine and manufacturer.
The corrective value and the result of the difference between the value of λ registered in the preceding step and a predetermined registered value of λ.
Document WO96/05419 comprises a weakening or an enriching of the mixture up to respectively verifying non-start-up or piston slap, which parameters are extraneous to the method of the present invention.